The candidacy of one of Canada’s biggest media magnates for the separatist Parti Québécois has thrown into question media independence and even the arrival of an eighth Canadian NHL franchise in Quebec City, Liberal Leader Philippe Couillard says.

Pierre Karl Péladeau’s tumultuous landing in the Quebec election campaign also raises questions about PQ Leader Pauline Marois’s judgment, Mr. Couillard said Tuesday as he campaigned in the provincial capital. Mr. Péladeau is the controlling shareholder of Quebecor, which owns dozens of newspapers across Canada and Quebec’s dominant private TV network.

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“It’s irresponsible of Ms. Marois to not have taken more precautions before adding to her team a person who, whatever his personal qualities, controls a large part of the written and electronic media of Quebec,” said Mr. Couillard.

Mr. Péladeau resigned Sunday from his corporate positions, but says he will hold on to his shares while putting his assets in a blind trust.

It’s not good enough, according to the Liberal leader.

“I don’t want to suggest to Mr. Péladeau what he has to do. But he has to tell us what influence he has had, and will have, and how he will guarantee to Quebeckers that during the campaign they will still be exposed to an independent impartial media,” he said.

Ms. Marois sought to reassure voters on Monday, saying Mr. Péladeau will be required to follow the same stringent conflict of interest rules overseen by the ethics commissioner as other ministers.

“I can assure you everything will be followed according to the letter of law. All the rules will be strictly followed. We now have an ethics commissioner to ensure that everyone complies,” Ms. Marois said.

If Mr. Péladeau is elected he will be required by law to place his shares in a blind trust. He will also have a duty to avoid placing himself in a conflict of interest. Given the extent of Mr. Péladeau’s holding, this may prove challenging especially if Peladeau is sworn in as a cabinet minister.

Ethic Commissioner Jacques St-Laurent said that Mr. Péladeau’s case could create a precedent. He explained that the law allows for publicly traded companies to continue doing business with the government even if shares belonging to an elected member are placed in a blind trust.

“There is no jurisprudence involving a situation like this. And it is impossible to rule on such a case without having full knowledge of all the assets involved and without knowing what the [ministerial] responsibilities will be,” Mr. St-Laurent said in an interview.

The only other case involving a potential conflict of interest of a elected member was with former Liberal labour minister David Whissell. Rather than sell his company, Mr. Whissell resigned from cabinet. Since Mr. Whissell was part owner of a privately owned paving company he was required by law to avoid doing business with the government. He quit after reports showed his company had received government paving contracts.

Numerous questions were raised about the influence Mr. Péladeau could continue to have over his powerful newspaper and television empire if he is elected and sworn in as a minister.

“Regardless if it is TVA, Journal de Montréal, Journal de Québec, Sun News, Toronto Sun, I never intervened in the news contents of their newsrooms,” Mr. Péladeau said.

Mr. Péladeau said his newsrooms work in total independence, but contrary accounts have long circulated in Quebec. Even PQ candidate Alexis Deschênes, who quit Quebecor’s TVA in 2010, has said he left journalism partly because his independence was undermined by directives from above. “Freedom of the press is being undermined. I fought daily battles,” he told Le Soleil just after tendering his resignation.

In Quebec City, where the Liberals compete with the right-leaning Coalition Avenir Québec for seats, Mr. Péladeau is highly regarded for spearheading an effort to secure a new publicly funded arena, and his so far unfulfilled attempt to bring an NHL team.

“It’s fundamentally a business transaction, and they’re mixing a business transaction with partisan politics, with a party that advocates the breakup of Canada. All of this together will not leave a very nice smell” with the NHL, he said.

“Quebec City has been putting money into getting an NHL team. I would see this as a threat. I’m not saying it’s going to cancel the project, but it’s definitely a negative.”

The steel frame of the $400-million arena is already rising next to Quebec City’s old Coliseum. The previous Liberal government and the city split the construction bill and handed control over to Quebecor in exchange for a $33-million payment, $3-million a year rent, and a promise of profit sharing. The initial payment rises to $63-million if an NHL team ever materializes.

Mr. Couillard’s reaction to Mr. Péladeau’s candidacy has been subdued and limited. On Sunday they left the field open to Mr. Péladeau’s harshest critics in the labour movement and the left wing, a campaign source said.

On Monday, Mr. Couillard said repeatedly that his message discipline will remain ironclad: His party stands for the economy, the PQ for a referendum.

“[Marois] will do everything she thinks is good to break up Canada and bring separation of Quebec. Anything,” he said.